1. Field of the Invention
The present invention relates to a zoom lens, and relates to a zoom lens suitable for an image pickup apparatus, such as a video camera, a digital still camera, a television camera, and a monitoring camera.
2. Description of the Related Art
It is necessary for zoom lenses used in image pickup apparatuses to have a wide angle of view, fast focusing capability, and a shallow depth of field for creating a soft focus effect to cause a main object to standout clearly against the background. On the other hand, it is necessary for image pickup apparatuses using image sensors to have sufficient image-side telecentricity to eliminate shading effects.
Downsizing of the image sensors makes it easier to realize the sufficient telecentricity of the zoom lens and downsizing of the image pickup apparatus at the same time, but it results in shorter focal lengths, and accordingly, deeper depths of field. If the image sensor is made larger with the telecentricity of the zoom lens kept sufficient, the entire zoom lens would be larger.
As a wide-angle zoom lens, a negative lead type zoom lens, in which a lens unit having negative refractive power is arranged nearest to the object side, is known.
Japanese Patent Application Laid-Open No. 11-72704 discusses a zoom lens which includes, in order from an object side to an image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, a third lens unit having positive refractive power, and a fourth lens unit having negative refractive power. In this zoom lens, each lens unit moves during zooming, and the first lens unit moves during focusing. Since the large and heavy first lens unit moves during focusing, it is difficult for the zoom lens to perform fast focusing.
Japanese Patent Application Laid-Open No. 2011-59293 discusses a zoom lens which includes, in order from an object side to an image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, and a third lens unit having negative refractive power. In this zoom lens, each lens unit moves during zooming. Japanese Patent Application Laid-Open No. 2011-59293 also discusses a configuration of dividing the third lens unit into a lens subunit 3a having negative refractive power and a lens subunit 3b having positive refractive power to move the lens subunit 3b during focusing.
Since this zoom lens has the third lens unit divided into the lens subunit 3a having negative refractive power and the lens subunit 3b having positive refractive power in order from the object side to the image side to ensure the telecentricity, the total optical system length and the effective diameter of the final lens tend to be larger. Further, since the zoom lens is configured on the assumption that negative distortion is to be corrected by image processing and has the lenses configured to allow optical distortion to be corrected, the zoom lens has relatively large fluctuation in curvature of field associated with focusing.
Japanese Patent Application Laid-Open No. 2006-208890 discusses a zoom lens which includes, in order from an object side to an image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, and a third lens unit having positive refractive power. In this zoom lens, each lens unit moves during zooming, and the third lens unit moves during focusing.
It is necessary for zoom lenses used in image pickup apparatuses to provide compactness of the entire zoom lens, a wide angle of view, easiness of fast focusing, and high optical performance over the entire object distance. To meet these requirements, it is important to appropriately set the lens configuration and refractive power of each lens unit.